The cost of producing and purchasing electricity has escalated to all-time highs worldwide. Such escalation is especially true in under-developed countries where electricity supply is limited, as well as in those countries with large populations where the demand for electricity is high. Driven by this demand is an ever-increasing desire to produce lighting sources that are energy efficient and minimize the cost of electric usage.
One of the more efficient lighting sources is the incandescent light bulb. Over the past two centuries, scientists and inventors have strived to develop a cost-effective, practical, long-life incandescent light bulb. Developing a long-life, high-temperature filament is a key element in designing a practical incandescent light bulb.
Tungsten filaments have been found to offer many favorable properties for lighting applications, such as a high melting point (3,410° C. or 6,170° F.), a low evaporation rate at high temperatures (10-4 torr at 2,757° C. or 4,995° F.), and a tensile strength greater than steel. These properties allow the filament to be heated to higher temperatures to provide brighter light with favorable longevity, making tungsten a preferred material for filaments in commercially available incandescent light bulbs.
The filament of an incandescent lamp emits visible and non-visible radiation when an electric current of sufficient magnitude is passed through it. The filament emits, however, a relatively small portion of its energy, typically 6 to 10 percent, in the form of visible light. Most of the remainder of the emitted energy is in the infrared region of the light spectrum and is lost in the form of heat. As a consequence, radiative efficiency of a typical tungsten filament, measured by the ratio of power emitted at visible wavelengths to the total radiated power over all wavelengths, is relatively low: on the order of 6 percent or less.
Conventional techniques for increasing the amount of visible light emitted by an incandescent filament rely on increasing the amount of energy available from the filament by increasing the applied electrical current. Increasing the current, however, wastes even larger amounts of energy. What is needed is a tungsten filament that emits increased visible light without increasing energy consumption.
Another concern is the life span of a filament. A tungsten filament is very durable. Nevertheless, after a prolonged period of time, large electrical currents cause excessive electron wind, which occurs when electrons bombard and move atoms within the filament. Over time, this effect causes the filament to wear thin and eventually break.
It has been observed that the radiative efficiency of filament material such as tungsten may be increased by texturing the filament surface with submicron-sized features. A method of forming submicron features on the surface of a tungsten sample using a non-selective reactive ion etching technique is disclosed by H. Craighead, R. Howard, and D. Tennant in “Selectively Emissive Refractory Metal Surfaces,” 38 Applied Physics Letters 74 (1981). Craighead et al. disclose that improved radiative efficiency results from an increase in the emissivity of visible light from the tungsten. Emissivity is the ratio of radiant flux, at a given wavelength, from the surface of a substance (such as tungsten) to radiant flux emitted under the same conditions by a black body. The black body is assumed to absorb radiation incident upon it.
Craighead et al. disclose that the emissivity of visible light from a textured tungsten surface is twice that of a non-textured surface. The authors suggest that the increase is a result of more effective coupling of electromagnetic radiation from the textured tungsten surface into free space. The textured surface of the tungsten sample disclosed by Craighead et al. has depressions in the surface separated by columnar structures projecting above the filament surface by approximately 0.3 microns.
Another method for enhancing incandescent lamp efficiency by modifying the surface of a tungsten lamp filament appears in a paper entitled “Where Will the Next Generation of Lamps Come From?”, by J. Waymouth, pages 22–25 and FIG. 20, presented at the Fifth International Symposium on the Science and Technology of All Light Sources, York, England, on Sep. 10–14, 1989. Waymouth hypothesizes that filament surface perforations measuring 0.35 microns across and 7 microns deep, and separated by walls 0.15 microns thick, may act as waveguides to couple radiation in the visible wavelengths between the tungsten and free space, but inhibit emission of non-visible wavelengths. Waymouth discloses that the perforations on the filament may be formed by semiconductor lithographic techniques, but such perforation dimensions are beyond current state-of-the-art capabilities.
Another method for reducing infrared emissions of an incandescent light source is described in U.S. Pat. No. 5,955,839 issued to Jaffe et al. As described, the presence of microcavities in a filament provides greater control of directivity of emissions and increases emission efficiency in a given bandwidth. Such a light source may have microcavities, for example, between 1 micron and 10 microns in diameter. Although features having these dimensions may be formed in some materials using microelectronic processing techniques, it is difficult to form them in the metals, such as tungsten, commonly used for incandescent filaments.
Yet another method for reducing infrared emissions of an incandescent light source is disclosed in U.S. Pat. No. 6,433,303 issued to Liu et al. and entitled “Method and Apparatus Using Laser Pulses to Make an Array of Microcavity Holes.” The disclosed method uses a laser beam to form individual microcavities in a metal film. An optical mask divides the laser beam into multiple beams and a lens system focuses the multiple beams onto the metal film and forms the microcavities. In their own research, the present inventors have used femtosecond laser pulses to drill holes on flat tungsten surfaces. Such laser drilling suffices to provide research samples, but laser drilling will not be suitable for mass production given the high cost of the drilling process. Moreover, drilling of curved, rather than flat, surfaces presents additional problems.
Still another method is disclosed in U.S. Pat. No. 5,389,853 issued to Bigio et al. Bigio et al. describe a filament having improved emission of visible light. The emissivity of the tungsten filament is improved by depositing a layer of submicron-to-micron crystallites on its surface. The crystallites are formed from tungsten or a tungsten alloy of up to 1 percent thorium and up to 10 percent of at least one of rhenium, tantalum, or niobium.
Although these conventional methods form microcavities and improve light emissivity, they are complex and costly. None of these methods is suitable for mass manufacturing environments where cost and efficiency are important factors. Consequently, a need still exists for a method of making microcavities in a filament that is suitable for mass manufacturing environments.